A globally recognized need to reduce sulfur levels in hydrocarbon streams such as gasoline and diesel fuels currently exists. The reduction of sulfur in such hydrocarbon streams may greatly improve air quality because of the negative impact sulfur has on performance of sulfur sensitive components such as automotive catalytic converters. The presence of oxides of sulfur in automotive engine exhaust may inhibit and eventually poison noble metal catalysts within catalytic converters and emission of those oxides of sulfur can have a negative impact on the environment. Emissions from inefficient or poisoned catalytic converters contain levels of many other undesirable materials, such as: non-combusted-non-methane hydrocarbons, oxides of nitrogen, and carbon monoxide. Such emissions may be photoconverted by sunlight generating ground level ozone, known also as smog.
Thermally processed gasolines such as, for example, thermally cracked gasoline, visbreaker gasoline, coker gasoline and catalytically cracked gasoline (hereinafter collectively referred to as “cracked gasoline”) contain, in part, olefins, aromatics, sulfur, and sulfur-containing compounds. Given that most gasolines, such as, automobile gasolines, racing gasolines, aviation gasolines, boat gasolines, and the like contain blends of, at least in part, cracked gasoline, reduction of sulfur in cracked gasoline will inherently facilitate reduction of sulfur levels in most gasolines, including: automobile gasolines, racing gasolines, aviation gasolines, boat gasolines, and the like.
There is a growing public recognition that lower sulfur gasoline reduces automotive emissions and improves air quality. Thus, the US Environmental Protection Agency rules to date have focused on the required level of reduction, the geographical areas in need of lower sulfur gasoline, and the time frame for implementation.
As the concern over the impact of automotive air pollution continues, it is clear that further effort to reduce the sulfur level in automotive fuels will be required. In 2008, the US Environmental Protection Agency standards will effectively require every blend of gasoline sold in the United States to meet a 30-ppm sulfur level.
In addition to the need to be able to produce low sulfur content automotive fuels, there is also a need for the implementation of systems and processes that will have a minimal effect on the olefin content of such fuels so as to maintain the octane number (both research and motor octane number). Such systems and processes would be desirable since saturation of olefins greatly affects the octane number. Such adverse effect on the olefin content is generally due to the severe conditions normally employed, such as during hydrodesulfurization, to remove thiophenic compounds (such as, for example, thiophenes, benzothiophenes, alkyl thiophenes, alkylbenzothiophenes, alkyl dibenzothiophenes and the like) which are some of the most difficult sulfur containing compounds to remove from cracked gasoline. In addition, there is a need to avoid systems and processes wherein the conditions are such that the aromatic content of the cracked gasoline is lost through saturation. Thus, there is a need for systems and processes that achieves desulfurization and maintains the octane number.
However, current processes may have adverse effects on the olefin content which may be generally due to the severe conditions normally employed, such as during hydrodesulfurization, to remove thiophenic compounds (such as, for example, thiophenes, benzothiophenes, alkyl thiophenes, alkylbenzothiophenes, alkyl dibenzothiophenes and the like). In removing sulfur from diesel fuels by hydrodesulfurization, the cetane number is typically improved; however there is a large cost in hydrogen consumption, since hydrogen is consumed by both hydrodesulfurization and aromatic hydrogenation reactions.
In addition to the need for removal of sulfur from cracked gasolines, there is also a need for the petroleum industry to reduce the sulfur content in diesel fuels. In general, it is much harder to remove sulfur from diesel fuel as compared to gasoline. Further, the high-pressure and high temperature required by hydro desulfurization requires expensive capital equipment infrastructure and high operating cost to achieve mandated low levels of sulfur.
Thus, there is a need for a desulfurization system and process without a significant consumption of hydrogen so as to provide a more economical process for the treatment of cracked gasolines and diesel fuels.
Some prior art catalysts include harsh acids, such as sulfuric acid, which are difficult to separate from the reaction mixture and have demonstrated incomplete conversion of desired reaction components. Transition metal catalysts are typically more facile to separate from the reaction mixture owing to their substantially different physical and chemical properties. Solid state heterogeneous catalysts are particularly facile to separate from liquid and gaseous reaction mixtures.
As a result of the lack of success in providing a successful and economically feasible process for the reduction of sulfur levels in cracked gasolines and diesel fuels combined with the fact that crude oil supplies are growing more sour (sulfur-rich) each day, it is apparent that there is a need for better catalyst systems and processes for the desulfurization of such hydrocarbon streams which have minimal effect on octane levels while achieving high levels of sulfur removal.
Thus, there exists a need for an economical and efficient catalytic desulfurization process for the treatment of sulfur-containing hydrocarbon streams, e.g., crude and crude oil distillates.